Conventionally, the high molecular weight polyesters used in films, sheets, fibers, and other molded articles were mostly aromatic polyesters such as polyethylene terephthalate and polybutylene terephthalate.
Aliphatic polyesters generally regarded as being biodegradable have attracted attention in recent years from the point of environmental protection and the like. As a method for producing the aliphatic polyesters, a method comprising directly esterifying an aliphatic dicarboxylic acid with an aliphatic diol or a method comprising conducting transesterification of an alkyl ester of aliphatic dicarboxylic acid or an anhydride thereof with an aliphatic diol to obtain a glycol ester or a low polymer thereof and then stirring the same by heat under high vacuum for polycondensation has been known.
Polymerization proceeds by removing a released component outside the system in the polycondensation reaction as above. It is generally conducted by distilling the released component from the system conditioned at a high temperature and under high vacuum. For example, Patent publication 1 discloses a method for producing an aliphatic polyester having an increased molecular weight comprising esterifying an aliphatic dicarboxylic acid with a glycol component and conducting a reaction to remove glycol from the formed polyester diol in the presence of a catalyst at a temperature of 180-230° C. under high vacuum at 0.005-0.1 mmHg.
However, it is not sufficient to put the system under high vacuum, but it is necessary to make the surface area of the reactants for polycondensation sufficiently large and effectively renewed. This is because the released component present on the surface of the reactants can be removed more easily. In particular, as the viscosity of the reactants is increased and it is difficult for the released component to diffuse from the reactants in the latter half of the polycondensation reaction, it is necessary to make the surface area of the reactants larger by mechanical stirring so as to effectively renew the surface. As the desired surface area and effective surface renewal cannot be achieved in using a conventional polymerization apparatus equipped with conventional stirring blades, problems where the reaction does not proceed sufficiently and an aliphatic polyester having an increased molecular weight cannot be obtained occur. Formation of a film or sheet using a polybutylene succinate having a low molecular weight is difficult.
In addition, as there is a limit to the direct polymerization described above, Patent publication 2 proposes preparation of an aliphatic polyester having an increased molecular weight by adding a diisocyanate having an isocyanate group in an amount one tenth to two times equivalent to the amount of the hydroxyl groups to an aliphatic polyester prepolymer substantially having terminal hydroxyl groups in a molten state at a temperature not lower than the melting temperature thereof. It has been known that the aliphatic polyester having an increased molecular weight obtained by this method has a weight average molecular weight (Mw) of 200,000 or higher that cannot be achieved by a conventional direct polymerization method and that formability and physical properties of films thereof are excellent. However, by this method, reduction of accuracy in controlling molecular weight and formation of gelation and fish eyes sometimes occurred when adding the isocyanate to a polymerization tank as in the conventional method.